The present invention relates to a method for producing a compact (i.e., green compact) of rare earth alloy powder, a rare earth magnet, and a powder compacting machine. More particularly, the present invention relates to a powder pressing method for a rare earth magnet that has a form requiring multi-stage filling and compacting of rare earth alloy powder.
When magnetic powder is filled in a cavity of a powder compacting machine (a press machine) and simply compressed, the magnetic moments of powder particles are only randomly oriented. If a magnetic field is formed in the cavity and magnetic powder filled in the cavity is compressed in the magnetic field, a compact with powder particles aligned in a desired direction can be produced. If the compact is made of rare earth alloy powder excellent in magnetic properties, a high-performance anisotropic magnet can be manufactured from the compact.
FIG. 1 illustrates a typical compacting machine used for the case of orienting magnetic powder particle in a radial direction. The machine in FIG. 1 includes a die 10 having a through hole, a magnetic core 12 having an outer circumference facing the inner wall of the through hole of the die 10, a cylindrical lower punch 14 inserted into the through hole of the die 10 from below, and a cylindrical upper punch 16 inserted into the through hole of the die 10 from above. The magnetic core 12 is composed of an upper core 12a and a lower core 12b that fit in core holes of the upper punch 16 and the lower punch 14, respectively. The upper core 12a and the lower core 12b are made of a ferromagnetic material, while the upper punch 16 and the lower punch 14 are made of a nonmagnetic material (e.g., core 12).
The die 10 shown in FIG. 1 has a layered structure composed of an upper portion made of a ferromagnetic material (magnetic portion 10a) and a lower portion made of a nonmagnetic material (nonmagnetic portion 10b). A cylindrical space is defined between the outer circumference of the core 12 and the inner wall of the magnetic portion 10a of the die 10. The cylindrical space can be blocked with the upper punch 16 and the lower punch 14 on the top and bottom sides thereof, respectively. The outer circumference of the core 12, the inner wall of the die 10, and top end face of the lower punch 14 form a xe2x80x9ccavityxe2x80x9d into which powder is filled. Magnetic powder 24 filled in the cavity is sandwiched by the upper punch 16 and the lower punch 14 and thus compacted by compression. In this case, the cavity is defined by the top end face of the lower punch 14, the outer circumference of the core 12, and the inner wall of the magnetic portion 10a of the die 10. A cylindrical sleeve 11 made of a nonmagnetic material may optionally be provided on the inner wall of the through hole of the die 10 to ensure that no step will be formed between the ferromagnetic portion and the nonmagnetic portion and that a compact will not be injured by such a step during removal from the die. In this case, the cavity is defined by the top end face of the lower punch 14, the outer circumference of the core 12, and the inner wall of the sleeve 11.
An upper coil 20 and a lower coil 22 are provided for forming a radial magnetic field inside the cavity. A magnetic field generated by the upper coil 20 and a magnetic field generated by the lower coil 22 repel each other in and around the center portion of the magnetic core 12, thereby forming a radial magnetic field that expands from the center portion of the core 12 radially toward the die 10. The arrows in FIG. 1 represent magnetic fluxes in the magnetic materials.
In order to improve the degree of alignment of magnetic powder in a compact to be produced, an intense radial magnetic field must be formed in the cavity. In order to increase the intensity of the radial magnetic field, it is desirable to increase electric power supplied to the coils 20 and 22, as well as optimizing the size and material of the core 12. However, increase in the electric power supplied to the coils will raise production cost and also cause a trouble of generating heat. Optimization of the size and material of the core is difficult because the core size is defined by the inner diameter of a magnet to be produced and improvement of the core material is limited.
In view of the above, when an axially elongated cylindrical magnet is to be manufactured, a multi-stage compacting process is employed where a powder filling step and a pressing step are repeated a plurality of times to ensure that an aligning magnetic field with a sufficient intensity is applied. In the multi-stage compacting process, when a long cylindrical compact is to be produced, a cycle of powder filling/compression in the magnetic field is repeated to sequentially produce axially divided portions of the compact. Accordingly, the cavity length per cycle is small and thus the intensity of the radial magnetic field formed in the cavity can be increased.
A conventional multi-stage compacting process will be described with reference to FIGS. 1, 2A and 2B.
First, as shown in FIG. 1, the magnetic powder 24 filled in the cavity is pressed in the presence of a magnetic field to produce a first-stage compact 26 (first-stage compression step). Thereafter, as show in FIG. 2A, magnetic powder 24 is filled in a cavity formed on the upper surface of the first-stage compact (denoted by 26) and pressed in the presence of a magnetic field (second-stage compression step). In the second-stage compression step, the cavity is defined by the top surface of the first-stage compact 26, the outer circumference of the core 12, and the inner wall of the magnetic portion 10a of the die 10. As show in FIG. 2B, by the second-stage compression step, a second-stage compact 28 is formed on the first-stage compact 26. The two compacts are integrated to form a compact 30.
By repeating the powder filling step and the compression step a plurality of times in the manner described above, an anisotropic ring magnet having a desired axial length can be manufactured beyond the limitation of the axial length L (see FIG. 1) of the magnetic portion 10a of the die 10. This method for manufacturing an anisotropic ring magnet by multi-stage compacting is disclosed in Japanese Laid-Open Publication No. 9-233776, for example.
The anisotropic magnet manufactured by the above conventional method has the following problem. Disorder in alignment arises at the boundary of the first-stage compact 26 and the second-stage compact 28, resulting in degradation in magnetization at the boundary.
FIG. 3 is a graph showing the surface magnetic flux density (Bg) at the outer circumference of a ring magnet (a cylindrical magnet) manufactured by the conventional multi-stage compacting method. The ring magnet manufactured and evaluated had an outer diameter of 16.4 mm, an inner diameter of 10.5 mm, and an axial length of 20 mm as measured after surface finishing. In the graph, the surface magnetic flux density (Bg) at the outer circumference of the magnet is shown by the solid line. The measurement was made using a gauss meter by scanning the surface of the magnet with a measuring probe. In the graph in FIG. 3, values in a region B correspond to values measured on the second-stage compact 28, while a values in a region C correspond to values measured on the first-stage compact 26.
FIG. 4 is a perspective view of the cylindrical magnet of FIG. 3, denoted by 32. The left-hand side of the magnet 32 (corresponding to the compact 30) in FIG. 4 corresponds to the upper portion of the compacting machine (upstream portion with respect to the pressing direction).
As is apparent from the graph in FIG. 3, a large drop in surface magnetic flux density (Bg) is observed at the boundary of the first-stage and second-stage compacts 26 and 28. Actually, the surface magnetic flux density (Bg) at the boundary is about 60% or less of the maximum value of the surface magnetic flux density (Bg) at the other portions.
The inventors of the present invention considered that the above local drop in magnetic flux density (Bg) was generated for the following reason. When the second-stage compression in the magnetic field is to be performed in the state where the first-stage compact 26 rests on the top end face of the lower punch 14 as shown in FIGS. 2A and 2B, magnetic fluxes leak to the first-stage compact 26 that is magnetic, resulting in generating distortion in the distribution of the radial magnetic field. This occurs because the magnetic field generated from the lower core 12b concentrates on and around the top surface of the first-stage compact 26 since magnetic fluxes pass through the first-stage compact 26 more easily compared with the rare earth alloy magnetic powder 24 filled for the second stage compression. In this way, magnetic fluxes shortcut to the magnetic potion 10a from the lower core 12b passing through the top portion of the first-stage compact 26 due to its high permeability, and as a result, distortion in the distribution of a radial magnetic field is generated significantly at and around the boundary of the first-stage and second-stage compacts 26 and 28. This means that the radial components of the aligning magnetic field decreases while the axial components thereof increases. If the number of axial components of the aligning magnetic field increases, the alignment of the magnetic powder 24 is disordered, resulting in lowering the degree of alignment.
If the distribution of the radial magnetic field formed in the second-stage compression step is disordered, the orientation of the powder is disordered not only in the second-stage compact 28 but also in the first-stage compact 26 even if disorder was small in the distribution of the radial magnetic field formed in the first-stage compression step. This is because particles are reoriented in an intense magnetic field such as that of 0.4 MA/m or more even after the magnetic powder 24 was already subjected to compression. If the magnetic powder 24 includes a lubricant, powder particles are likely to rotate more easily. In this case, therefore, the orientation or alignment of the first-stage compact 26 is further disordered. As the magnetic field applied in the second-stage compression step is greater, the degree of alignment of the first-stage compact 26 is more lowered.
The lowering in the degree of alignment is considered more likely when a sintered magnet is manufactured than when a bonded magnet is manufactured. This is because, when magnetic powder is compacted for sintering, the compression density of the powder is made comparatively small. The resultant first-stage compact 26 is more susceptible to a disordered magnetic field due to this reduced compaction.
The conventional method has another problem as follows. When a compact produced by the multi-stage compacting method is sintered, the resultant sintered body is poor in size precision. The reason is that rare earth alloy powder used for manufacturing a rare earth sintered magnet is markedly poor in flowability if granulation (machining of powder) is not performed. It is difficult to fill such powder in the cavity at a uniform density. In addition, it is difficult to feed a dispensed amount of powder to a cavity if the cavity is of a cylindrical shape. Therefore, a feeder box containing powder in an amount far exceeding the amount to be filled is moved to the position above the cavity, where the powder is allowed to fall freely and the powder filled in the cavity is wiped off with a bottom edge of the feeder box. This causes variation in filled amount of the powder. In the conventional pressing, the operations of the die and punches are controlled on the presumption that the filling density of powder in the cavity is uniform. The positions of the die and punches during compression invariably follow predetermined position settings. Therefore, if a variation exists in the filling density of powder, the density of the resultant compact varies, and thus the shrinkage rate of the compact during sintering varies. As a result, the size of the sintered body varies both in the compacting direction (height direction) and the thickness direction.
A primary object of the present invention is providing a method for producing a compact of rare earth alloy powder capable of producing a high-quality compact where local drop in the degree of alignment is suppressed even in the multi-stage filling and compacting process.
Another object of the present invention is providing a permanent magnet having an improved magnet properties obtained from a radially aligned compact produced by the above compacting method.
The method for producing a compact of rare earth alloy powder of the present invention uses a compacting machine including: a die including a nonmagnetic portion and a magnetic portion placed on the nonmagnetic portion, the die having a through hole; a magnetic core having an outer circumference facing an inner wall of the through hole; a lower punch for being inserted from below into a space formed between the inner wall of the through hole and the outer circumference of the magnetic core; and an upper punch for being inserted from above into the space formed between the inner wall of the through hole and the outer circumference of the magnetic core. The method comprising: a powder-filling step comprising filling rare earth allow powder in a cavity formed by inserting the lower punch into the through hole; and a compression step comprising pressing the rare earth alloy powder while applying a magnetic field to the rare earth alloy powder, the powder-filling and compression steps being repeated a plurality of times. When an (n+1)th stage compression step is to be carried out, where n is an integer equal to or greater than 1, a top surface of a compact produced in an n-th stage compression step is placed at a position above a bottom surface of the magnetic portion of the die.
Alternatively, the method for producing a compact of rare earth alloy powder of the present invention includes: a powder filling step of filling rare earth alloy powder in a cavity formed in a space between a first magnetic member and a second magnetic member; and a compression step of pressing the rare earth alloy powder while applying a magnetic field, the steps being repeated a plurality of times. When an (n+1)th (n is an integer equal to or more than 1) stage compression step is to be carried out, at least part of a compact produced in an n-th stage compression step is placed in the space between the first magnetic member and the second magnetic member.
The intensity of the magnetic field in the cavity is preferably 0.4 MA/m or more.
A lubricant may be added to the rare earth alloy powder.
Preferably, the amount of the rare earth alloy powder filled in the cavity is larger in an n-th stage powder filling step than in an (n+1)th stage powder-filling step.
Preferably, in the (n+1)th stage compression step, the level difference between the top surface of the compact produced in the n-th stage compression step and the bottom surface of the magnetic portion of the die is 3 mm or more.
Preferably, in the (n+1)th stage compression step, the height of the part of the compact produced in the n-th stage compression step placed in the space is 3 mm or more.
In a preferred embodiment, the rare earth alloy powder is made of a R-T-(M)-B alloy (where R denotes a rare earth element containing at least one kind of element selected from Y, La, Ce, Pr, Nd, Sm, Gd, Th, Dy, Ho, Er, Tm, and Lu; T denotes Fe or a mixture of Fe and Co; M denotes an additive element; and B denotes boron).
Preferably, the compact is of a cylindrical shape, and the magnetic field is a radial magnetic field.
The density of the compact produced in the n-th stage compression step is preferably 3.5 g/cm3 or more.
In a preferred embodiment, the compression step of pressing rare earth alloy powder while applying a magnetic field includes a step of measuring the pressure applied to the rare earth alloy powder filled in the cavity.
Preferably, the density of the compact produced in the compression step is adjusted by controlling the pressure applied to the rare earth alloy powder.
The method for manufacturing a rare earth magnet of the present invention includes sintering, to obtain a permanent magnet, a compact produced by any method for producing a compact of rare earth alloy powder described above.
The rare earth magnet of the present invention is manufactured by repeating a plurality of times a powder filling step of filling rare earth allow powder in a cavity and a compression step of pressing the rare earth alloy powder while applying a magnetic field. The surface magnetic flux density at a boundary of an upper compact produced in an (n+1)th (n is an integer equal to or more than 1) stage compression step and a lower compact produced in an n-th stage compression step is 65% or more of the maximum value of the surface magnetic flux density at the other portions.
The powder compacting machine of the present invention includes: a die including a nonmagnetic portion and a magnetic portion placed on the nonmagnetic portion, the die having a through hole extending through the nonmagnetic portion and the magnetic portion; a magnetic core having an outer circumference facing an inner wall of the through hole of the die; a lower punch for being inserted from below into a space formed between the inner wall of the through hole of the die and the outer circumference of the magnetic core; an upper punch for being inserted from above into the space formed between the inner wall of the through hole of the die and the outer circumference of the magnetic core; a powder feed device for filling magnetic powder in a cavity formed by inserting the lower punch into the through hole of the die; a magnetic field generator for applying a magnetic field to the magnetic powder filled in the cavity; a first controller for controlling relative positions of the die and the lower punch; and a second controller for controlling relative positions of the upper punch and the lower punch. The powder compacting machine operating to repeat a powder-filling step comprising filling magnetic powder in the cavity and a compression step comprising pressing the magnetic powder while applying the magnetic field to the magnetic powder. The first controller controls the relative positions of the die and the lower punch so that when an (n+1)th stage compression step is to be carried out, where n is an integer equal to or greater than 1, a top surface of a compact produced in an n-th stage compression step is placed at a position above a bottom surface of the magnetic portion of the die.
In a preferred embodiment, the powder compacting machine further includes a pressure sensor for measuring a pressure applied to the magnetic powder.
Preferably, the pressure sensor includes a strain gage adapted for detecting strain on the upper punch or the lower punch.
Preferably, the second controller controls the relative positions of the upper punch and the lower punch according to the pressure detected by the pressure sensor.
Alternatively, the method for producing a compact of rare earth alloy powder of the present invention includes: a first cavity formation step of forming a first cavity defined by a die and a lower punch; a first powder-filling step of filling rare earth alloy powder in the first cavity; a first compression step of compressing the powder filled in the first cavity until a pressure applied to the powder in the first cavity reaches a predetermined value; a second cavity formation step of forming a second cavity on the compressed powder by relative movement of the die and the lower punch after the first compression step; a second powder filling step of filling rare earth alloy powder in the second cavity; and a second compression step of compressing the powder filled in the second cavity until a pressure applied to the powder in the second cavity reaches a predetermined value.
In a preferred embodiment, the method further includes a storing step of storing the position of a top surface of the compact produced in the first compression step, and the second cavity formation step includes forming the second cavity by the relative movement of the die and the lower punch based on the position of the top surface of the compact.
Preferably, the first cavity and the second cavity are of a cylindrical shape.